1. Field of the Invention
The present invention generally relates to range gate imaging and more specifically, to a duo-frame normalization technique and system for use in range gate imaging to determine a range coded image.
2. Description of Prior Art
Range gate imaging involves sending out a narrow light pulse or train of pulses that is received by gating a receiver open and closed at a desired moment. The time to gate the receiver is determined by the round trip time the light pulse(s) must travel to return from the range being imaged. When the light pulse(s) from the desired range has returned to the receiver the receiver is shuttered to allow just that pulse in. This technique has been utilized as a method to negate low visibility atmospheric effects so as to determine the range of a target and to produce a composite range coded image for an entire range so as to aid in identification of a target.
Light-scattering particles present in an atmosphere can completely swamp the returning signal such that the resulting image does not convey useful information. By gating the signal, light which returns from objects or atmospheric scatters closer or farther away than the predetermined range is thus rejected. A range coded image for an entire range can also be derived from gating range imaging so as to identify what is the object of interest. A single imaging frame represents one range bin where the range width/precision of the bin is determined by the pulse width and receiver shutter speed (the receiver is usually gated for a time equal to the pulse width). To provide adequate range coverage of a scene many frames are required. For example, to provide range coverage from 50 to 500 meters with 5 meter wide range bins, it would require at least 90 frames which translate into a total of 3 seconds for a system with a 30 hertz frame rate. This lengthy acquisition time severely limits the real time utility of conventional range gate imaging.
Sequential image collection is also subject to frame to frame fluctuations due to phenomenological issues including those associated with: atmospheric turbulence, speckle, laser mode stability and platform motion (jitter). Atmospheric turbulence can significantly change the spatial distribution of energy in a laser beam. Refractive index variations along the laser propagation path thereby lead to an interference pattern of randomly distributed bright and dark regions in the beam at some distance from the laser. Another interference pattern results at the receiver due to reflection of radiation from a rough surface as each point on the surface reflects with a random phase shift. A speckle pattern thus often is seen as a grainy structure in laser based photography. Further not all lasers produce the same beam distribution every time they are fired. Multi-mode lasers can have significantly different beam profiles from one pulse to another, thus the variation will also degrade the processed range image of a sensor. Platform motion can change the registration of pixels on the target from one frame to the next. Even sub-pixel motion can adversely affect the quality of the processed image.
While the prior art has reported using range gate imaging none have established a basis for a specific apparatus that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a new technique to obtain a range coded image by range gating imaging with a short acquisition time. A new technique to obtain a range coded image by range gating imaging is also needed that substantially compensates for phenomenlogical issues.